The interaction between light and a hole in an opaque screen has been the object of curiosity in technology application for centuries. Grimaldi (F. M. Grimaldi, Physico-Mathesis De Lumine, Coloribus, et Iride, 9, 1665) first described diffraction from a circular aperture thereby providing an experimental basis for classical wave optics in the 17th century. Conventional diffraction theory of light transmission through a sub-wavelength aperture predicts a strongly attenuated transmitted beam (H. A. Bethe, Phys. Rev. 66, 163, 1944; T. W. Ebbesen, et al., Nature (London) 391, 667, 1998).
However, an extraordinary transmission phenomenon is seen to take place when light interacts with an array of sub-wavelength apertures in an opaque metal sheet. In 1998, Ebbesen, et al. made the remarkable observation of transmission efficiency from sub-wavelength circular apertures which was orders of magnitude greater than predicted by a standard aperture theory. Experiments provided evidence that the unusual optical property was due to the coupling of light with plasmons on the surface of the periodically patterned metal film. It was also observed that arrays of such holes display highly unusual zero-order transmission spectra at wavelengths larger than the array period beyond which no diffraction occurs. In addition, sharp peaks in transmission were observed at wavelengths as large as 10 times the diameter of the cylindrical apertures.
It is believed that light incident on a metal thin film establishes oscillations in the mobile charge density (ripples in the “Fermi sea”). These ripples, or plasmon excitations in the metal foil give rise to an evanescent mode of re-radiation that has been used in the past for contact printing. In addition, the ripples also excite the cavity modes of circular apertures in the thin film. These cavity modes act as intense light sources propagating into the far-field, drawing energy from their surroundings on which light is incident. The net transmission is far greater than the aperture area would dictate if taken alone.
It would be highly desirable to use the discovery of the plasmonic effect and extraordinary transmission phenomenon of the light interaction with an array of sub-wavelength holes formed in an opaque metal sheet to provide inexpensive ultra-high resolution sub-wavelength lithographic system for fabrication of semiconductor integrated circuits, data storage, as well as in microscopy, bio-photonics, etc.
Referring to FIG. 1, a conventional mask 10 for photolithography has a continuous clear pattern 12 formed in an opaque mask plate 14. The clear region 12 of the conventional mask is formed at a location, and sized and shaped to permit “imprinting” of a micro- or nano-feature on a substrate (wafer) as it is conventional in the photolithography. In order to “write” a pattern on the mask, a focused ion beam (or electron beam) is scanned over the mask addressing all pixels corresponding to the clear region. In this mask, the fill-factor, e.g. the ratio of the clear-to-opaque area is quite large. The necessity to expose all pixels of the opaque mask plate corresponding to the clear region requires a lengthy writing process for the conventional mask. Therefore, it would be highly desirable to reduce the “writing time” in the mask fabrication.